SCHOTT Technical Glasses

18
Sealability
In fusion sealing with other materials, the decisive glass
property is the linear contraction. As Figure 14 shows, the
experimental setting point T E lies in the already sharply bent
section of the glass curve, and the experimental setting
temperature increases with increasing cooling rate. Predicting
the linear contraction is only possible if the shape of
the glass curve and the setting point T E for the respective
cooling rate are known. The stress-optical measurement of
stresses in test fusions with practice-oriented cooling rates
(ISO 4790) is a simpler and much more accurate method
of testing the sealability.
Thermal stresses
Owing to the low thermal conductivity of glass (typically
0.9 – 1.2 W/(m · K) at 90 °C, or a minimum of 0.6 W/(m · K)
for high-lead-containing glasses), temperature changes produce
relatively high temperature differences ΔT between
the surface and the interior, which, depending on the elastic
properties E (Young’s modulus) and µ (Poisson’s ratio), and
on the coefficient of linear thermal expansion α, can result
in stresses
In addition to the geometric factors (shape and wall thickness),
the material properties α, E and µ decisively influence
the thermal strength of glasses subjected to temperature
variations and/or thermal shock. Thermal loads of similar
articles made from different glasses are easily compared by
means of the characteristic material value
φ = σ
ΔT = α E
1 – µ [MPaK–1 ],
which indicates the maximum thermally induced stress to be
expected in a flex-resistant piece of glass for a local temperature
difference of 1 K. Because cracking originates almost
exclusively from the glass surface and is caused there by tensile
stress alone, cooling processes are usually much more
critical than the continuous rapid heating of glass articles.
σ = ΔT α E
(1 – µ) [MPa].
300
Compressive
stress
200
0
Tensile
stress
100 100 MPa
x
a
c
A
B
C
M
b
M
Relative contraction ΔI/I ––>
Tg
x
x
0 200 400 600 800 1000
Temperature in K ––>
Fig. 11. Stress distribution across the thickness of thermally prestressed
flat glass (a) without, (b) with additional bending M; (c) stress distribution
in bending without prestressing
Fig. 12. Typical thermal expansion – temperature curve for glasses